GZ will bring you to an example of the basalt that makes up this region, please be very careful here as it can get very slippery. During the dry season you will be able to see bare rock that is usually behind the waterfall. Follow safety warnings at all times.
TO LOG THIS CACHE
To claim a find please email your answers to me via the GC.com website (PLEASE DO NOT POST YOUR ANSWERS IN YOUR LOG). There is no need to wait for confirmation before logging, if there is a problem with your answers I will get back to you.
1) Describe the surface of the rocks at GZ. Are they smooth or rough? Why do you think they might be this way, what caused it? YOU DO NOT HAVE TO APPROACH THE EDGE TO GET THESE ANSWERS
2) Did these rocks go through an intrusive or extrusive process when they formed? What are the differences in these types of rocks once cooled?
3) Take and post an optional picture of yourself or your GPS at this spot or anywhere else at the falls.
WHERE THE BASALT CAME FROM
150-200 million years ago during the Jurassic or Upper Karoo Period, when the present-day land masses forming South America, African, India, Australasia and Antarctica were all joined as one huge super-continent known as Gondwanaland, tectonic activity caused a break up of this landmass, with huge cracks or fissures forming in the earth's crust from which erupted huge volumes of molten basalt.
From Kazungula on the Botswana border to the Matetsi river, an isolated area of this basalt remains exposed, over which the present-day Zambezi flows and has cut its path, forming the falls and gorges. see Earthcache 'Great River' to learn more about the formation of the Zambezi River. The depth of the basalt is not known, perhaps up to 1,000 metres - but at least 300 meters in depth at the Falls region.
As the molten eruptions slowed and stopped, and the region stabilised, the central area of southern Africa subsided, perhaps under its own weight, to form what is often described as an inverted dish effect, with a raised border around most of the coastal areas and a shallow depressed central area forming a vast internal drainage network across what is known as the Southern African Plateau. Most of this ancient plateau is now heavily eroded, but in areas the flat-topped hills which are left by the erosive processes are isolated remnants of the original plateau.
After this period, there is a gap of more than 100 million years in the geological record. Any deposits that may have formed on top of the basalt during this time were later completely eroded away.
Much later, new deposits began to accumulate. Today, the area is covered by a thick layer of loose, reddish Kalahari sand, formed under desert conditions and carried by wind across much of southern Africa.
These sands border the broad, shallow Zambezi Valley above Victoria Falls, where the river flows over exposed basalt. Downstream of the Falls, the valley continues at a similar level, while the river has cut a deep gorge into the ancient basalt riverbed.(See EarthcacheThe Cataract for more information on the gorges)
WHAT IS BASALT
Basalt is a volcanic rock formed when molten lava reaches the surface through a volcano or, in this case, through fissures in the Earth’s crust. It is typically a dark grey to almost black, fine-grained rock.
Large layers of basalt can form when enormous quantities of lava rise through cracks in the crust and spread across the surface. As the lava cools rapidly, many small crystals begin to form but do not have enough time to grow larger. This gives basalt its characteristic fine-grained texture.
Although fresh basalt is generally dark grey or black, weathering can gradually change its appearance. Iron-rich minerals within the rock oxidise when exposed to air and water, producing brown and rust-red colours. As a result, weathered basalt can display a much wider range of colours than freshly formed basalt.
USES
Basalt is most commonly used during construction projects. It can be used as aggregate in asphalt and concrete pavements. It is also used as a road base, railroad ballast and cobblestone pavement. Basalt may be cut into thin slabs and used as monuments, building veneers or floor tiles.
Be aware that the river flow varies from month to month and depends of the rainfall that has occurred in the upper Zambezi. The graph below gives you an idea of the average flow rate and when best to view the falls.
During the dry season the view is more likely to look like this.